The present invention relates to a lens driving device employed in cameras and/or video cameras for driving a lens to move. More particularly, the present invention relates to a lens driving device for controlling the movement of a focusing lens, which can be moved manually even when it is moved for an automatic focusing.
In an AF (automatic focusing) system for a camera, a photographing lens, with which an automatic focusing operation can be performed, is controlled such that a position, along an optical axis, of a focusing lens (which may be a group of lens elements) included in the photographing lens is detected, and then the focusing lens is controlled to move to an in-focus position based on a distance to an object that is measured by a distance measuring device of the camera.
In an example of such an AF system, in order to detect the position of the focusing lens, an actuator which outputs a pulse signal in response to the movement of the focusing lens is provided. By counting the number of pulses included in the pulse signal, the lens position is detected.
Such an actuator is generally provided with a focusing motor, and a focusing cam ring, which is rotated by the focusing motor. The focusing cam ring is formed with cam grooves, to which cam followers provided to the focusing lens (or a lens barrel thereof) are engaged. As the focusing cam ring is rotated by the focusing motor, the focusing lens is driven to move along the optical axis thereof due to the engagement of the cam grooves with the cam followers. The actuator is further provided with a pulse encoder which generates a pulse signal in response to the rotational movement of the focusing cam ring.
When the focusing lens is driven to start moving, the moving speed of the focusing lens is controlled by controlling the rotation speed of the focusing motor so that the focusing lens quickly starts moving and smoothly. When the focusing lens is to be stopped, the moving speed thereof is controlled so that the focusing lens does not run past a destination, nor stop in front of the destination.
For example, at an initial stage of the movement, the focusing motor is driven to rotate at a relatively slow speed, and the speed is accelerated until it reaches a predetermined constant speed. Thereafter, the motor is controlled to keep rotating at the predetermined constant speed. When the lens approaches the destination, or a target position, the rotation speed of the focusing motor is gradually decelerated, and finally, the focusing motor is stopped.
Recently, a photographing lens allowing a user to manually move the focusing lens even when the automatic focusing is performed (i.e., the focusing lens is being moved by the AF system) has been developed. Example of such a lens is described in Japanese Patent Provisional Publication No. HEI 5-215954. In this publication, a lens includes an AF actuator for the automatic focusing, and a manually operable ring for manual focusing. Both of them are connected to a focusing cam ring through a frictionally connecting mechanism. With this construction, even when the focusing lens is moved by the AF actuator, by operating the MF ring, the user can move the focusing lens manually. In such a system, if the focusing lens is manually moved when it is moved by the AF system, correlation between the rotation speed of the focusing motor and moving amount of the focusing lens is changed, and therefore, it becomes impossible to control the moving speed of the focusing lens appropriately.
For example, a case in which the focusing lens is started to move and accelerated will be considered. It is assumed that the pulse signal, which is referred to and used for controlling the movement of the focusing lens, from the focusing cam ring is used as moving speed data, and the focusing cam ring is to be rotated at a certain target speed.
If the MF ring is rotated in the same direction as the focusing cam ring, the speed of the focusing motor is lowered in comparison to a case where the MF ring is not operated. For example, when the focusing motor is driven such that the focusing lens moves toward an object, if the MF ring is operated to move the focusing lens in the same direction, the encoder outputs the pulse signal having a higher frequency. Therefore, the AF actuator lowers the rotation speed of the focusing motor so as to lower the rotation speed of the focusing cam ring. If the MF operation is abruptly stopped under this condition, the speed of the focusing cam ring has become slower than the expected speed since the focusing motor has been lowered.
If the MF ring is operated in the direction opposite to that of the focusing motor, the speed of the focusing motor is higher than a case where the MF ring is not operated. If the MF ring is abruptly stopped under this condition, since the focusing motor is rotated at the higher speed at that instance, the focusing cam ring is moved at a speed higher than expected. As a result, if the MF ring is moved and abruptly stopped when the AF operation is being executed, the rotation speed of the focusing motor becomes unstable, and therefore, it becomes difficult to move the focusing cam ring at a predetermined speed to a target position quickly.
Next, a case where the focusing lens is decelerated and stopped will be considered. It is assumed that the pulse signal from the focusing cam ring is used as the moving speed data, and the speed of the focusing motor is to be lowered to a certain target speed.
If the MF ring is rotated in the same direction as the focusing cam ring rotates, the focusing motor is driven to run at a slower speed than the speed when the MF ring is not operated. If the MF ring is operated in the direction opposite to that of the focusing motor, the focusing motor is rotated at a higher speed than that when the MF ring is not operated. If the MF ring is abruptly stopped when the focusing motor is being rotated, the rotation speed of the focusing motor becomes unstable, and therefore, it becomes difficult to move the focusing cam ring at a predetermined speed to a target position quickly.
Alternatively, the pulse signal from the focus ring may be used as moving amount information, and the movement of the focusing ring can be controlled based on the moving amount information. In such a case, however, if the MF ring is rotated in a direction opposite to that of the focusing motor, rotation of the MF ring cancels the rotation of the focusing motor, which temporarily stops the rotation of the focusing motor. If the MF ring is rotated such that load to the focusing motor is decreased, the focusing motor becomes rotatable even if it should not be moved. If the MF ring is abruptly stopped, since the driving torque of the AF actuator runs short, the focusing motor is stopped. Then, no pulse is generated, and it becomes impossible to perform the AF control.